1. Field of the Invention
The invention is directed to a method and apparatus for load following with a single-cycle boiling moderator-coolant nuclear reactor.
2. Description of the Prior Art
Several different types of nuclear power reactors, using the heat energy produced by nuclear fission reactions to perform useful work, have been developed. Reactors of the single-cycle boiling moderator-coolant type, such as boiling water reactors, hereinafter referred to as BWR's have been found to be highly desirable for many applications. A typical reactor of this type includes a chain reacting assembly or core made up of nuclear fuel material contained in fuel elements. Fuel material is encased in a corrosion-resistant heat conductive shell or cladding. The reactor core, made up of a plurality of these elements in spaced relationship, is enclosed in a pressure vessel through which the reactor moderator-coolant flows. As the coolant passes between the spaced fuel elements, it is heated and evaporated by the energy given off during the fission reactions. The resulting steam leaves the reactor, is directed to a fluid-driven power system such as a turbine generator system, is condensed and is finally recycled back to the reactor.
In such reactors, the coolant acts to remove heat from the reactor core, to slow down or moderate fast neutrons released through fission events in the fuel, to increase the probability of an occurrence of subsequent fissions and to maintain a chain fission reaction. As boiling occurs within the core, the formation of vapor bubbles in the coolant decreases the amount of liquid moderator in the reactor core, thereby decreasing reactivity. Thus, increased reactivity in the core tends to increase heat generation forming more vapor bubbles. These bubbles, in turn, tend to decrease reactivity. In this manner the reactor is self-regulating. The general reactivity level in such a reactor is set by adjusting the control rods. For example, if the control rods are partially withdrawn, neutron flux level and, therefore, reactivity increases. The increased reactivity increases heat generation which causes formation of additional vapor bubbles. As the vapor bubbles form, the moderation effect of the coolant decreases, compensating partially for the increased reactivity. Boiling will continue at this level so long as reactor pressure remains constant. However, if pressure is changed substantially during operation without compensatory adjustments being made in the reactor power level, the reactor may not be self-regulating since an increase in pressure tends to inhibit formation of moderator vapor bubbles, permitting the power level to increase. Therefore, it is preferable that the reactor internal pressure be maintained substantially constant and that reactivity be controlled by other means. The basic reactor control system consists of the control rods containing neutron absorbing materials which decrease reactivity when inserted. However, the change in the reactor power level and steam output resulting from a change in control rod position is a relatively long-term change generally taking place over a 20-30 second period. Such a delay in the change in steam output is generally undesirably slow in practice where considerably faster responses are desired.
BWR's have an additional control mechanism available which is not available to other types of reactors. They have a moderator-coolant recirculation flow that is pumped from the reactor from a point above the reactor core, and pumped back into the reactor below the core. By varying this moderator-coolant recirculation flow, the quantity of liquid coolant per unit time passing through the core may be varied. Thus, if an increase in power level is desired, recirculation flow can be increased thereby sweeping vapor bubbles out of the core at a more rapid rate. Since the proportion of the core containing liquid rather than vaporized coolant increases, the moderation effect increases and thus reactivity increases. Where it is desired to decrease the power level of the reactor, recirculation flow may be decreased, thereby sweeping the vapor bubbles out of the core at a lower rate. Since the core will then contain a higher proportion of vapor bubbles and a lower proportion of moderating liquid coolant, reactivity will decrease. Therefore, after the basic reactor operating level desired is set by adjusting the control rods the reactor power level can be varied over a substantial range by varying the coolant recirculation flow rate to follow the load steam requirements.
While the reactor will respond to a change in steam demand from the load more rapidly by adjusting the recirculation flow rate than by simply adjusting the control rod positions, there is still a delay of about 5 to 15 seconds before the new power level is reached. Thus, there is a continuing need for an improved BWR control system which would respond more rapidly to changing demands.
This delay in response has heretofore been considered inherent to the BWR because of the low heat storage, or heat capacitance of the BWR and the aforementioned problem of maintaining a relatively constant reactor pressure. Pressurized moderator-coolant type nuclear reactors, such as pressurized water reactors, hereinafter referred to as PWR's, and dual-cycle BWR's do not have this problem because of the high heat storage capacity of these types of reactors. These types of reactors have a large heat storage capacity in a steam generator in the secondary cycle of the reactor which transforms heat from a primary flow of moderator-coolant water into steam for driving a turbine. The secondary cycle of these types of reactors also serves to isolate the reactor from pressure fluctuations related to power demands. Thus, PWR's and dual-cycle BWR's are capable of responding to daily load following and frequency regulation changes in demand on direct turbine governor control whereas BWR's generally have a turbine governor slaved to a slower responding reactor. Of course this restriction on responding to load following or frequency regulation changes in demand is particularly onerous to the operator of a nuclear power plant that must meet widely varying demands.
The prior art reveals solutions to this heat capacitance problem that are satisfactory for small PWR's small dual-cycle BWR's and fossil fuel plants having one-pass boilers with a low heat capacitance. For example, in U.S. Pat. No. 3,457,725 to A. Schwarzenbach an apparatus is revealed which throttles or closes a secondary steam flow from the steam supply system to increase steam flow through the turbine and thereby meet peak demands or rapidly changing demands. The secondary steam flow from the steam supply system is used to heat plant auxiliary equipment such as reheaters and feedwater heaters. In this way Schwarzenbach employs the heat capacitance of this auxiliary equipment to cover a peak demand or a rapidly changing demand. However, the apparatus revealed by Schwarzenback is particularly unsuitable for use with a BWR because secondary steam flow is throttled to increase steam flow through the turbine without regard to the effect on the pressure of the steam in the steam supply system. Such an uncontrolled throttling of a secondary supply of steam can cause pressure fluctuations in the steam supply system. This would be unsuitable in a BWR since if the pressure of the nuclear steam supply system of a BWR is not maintained constant the reactor may not be self-regulating as previously discussed. Sudden increases in pressure inhibit the formation of moderator vapor bubbles causing a flux excursion or reactor power increase. Sudden decreases in pressure cause an increase in the formation of moderator vapor bubbles resulting in a sharp reduction in reactor power level.
Others have suggested the throttling of different sources of secondary steam in the power plant. For example, in U.S. Pat. No. 3,411,299 to F. Nettle it is suggested that extraction steam from the turbine may be throttled to improve peak load operation in a steam power plant. However, the arrangement revealed by Nettle suffers from the same disadvantages pointed out above with respect to the Schwarzenbach apparatus since Nettle throttles extraction flow to increase the output of the turbine without regard to the effect on the pressure of the steam supply system.
Others have suggested the addition of a steam accumulator or a fossil fuel source of heat to the steam supply system to provide the necessary heat storage to allow the turbine generator to respond to daily load following or frequency regulation changes in demand. However, this approach is prohibitively complex and expensive.
Others have suggested that a turbine by-pass may be employed to by-pass unwanted steam to the condenser. For example, see U.S. Pat. No. 3,128,233 to R. E. Kuerzel. However, the prior art has not considered the combination of this feature with the ability to throttle secondary flows of steam in a load following system that solves the particular problems associated with achieving rapid load following with a BWR.
Since a large number of BWR's are already in commercial use and a large percentage of the grid capacity of some electrical utilities is comprised of BWR's, there is also a need for an improved BWR load following system which may be retrofitted to existing plants.
It is therefore an object of the invention to provide a load following system for a BWR having fast initial response to rapidly changing demands.
It is another object of the invention to provide a load following system for a BWR that allows direct governor control of the turbine.
It is another object of the invention to provide a load following system for a BWR that will allow a BWR to respond to rapidly changing demands as fast or faster than a PWR.
It is another object of the invention to provide a load following system for a BWR that allows fast initial response to rapidly changing demands without a change in reactor pressure.
It is another object of the invention to provide a load following system for a BWR that is capable of fast initial response to rapidly changing demands without causing a change in reactivity or a reactor power fluctuation.
It is another object of the invention to provide a load following system for a BWR that is simple, inexpensive and that may be retrofitted to existing BWR's.